1. Field of the Invention
The present invention relates to an energy-trap type piezoelectric resonator and more particularly, to an energy-trap type piezoelectric resonator including a substantially rectangular piezoelectric plate and vibrating using a fundamental wave of a thickness shear vibration mode.
2. Description of the Related Art
Energy-trap type piezoelectric resonators utilizing a thickness shear vibration mode have been used as resonators and filters. In FIGS. 6A and 6B, a perspective view and a vertical sectional view of a conventional piezoelectric resonator are shown.
A piezoelectric resonator 51 contains a long and narrow rectangular piezoelectric plate 52. The piezoelectric plate 52 is made of a piezoelectric ceramic material, such as lead zirconate titanate based ceramic, and is polarized in the longitudinal direction.
A resonance electrode 53 is disposed on the upper surface of the piezoelectric plate 52 and a resonance electrode 54 is disposed on the lower surface. The resonance electrodes 53 and 54 are opposed to each other with the piezoelectric plate 52 disposed therebetween in the middle of the longitudinal direction of the piezoelectric plate 52. The resonance electrode 53 extends to one end in the longitudinal direction of the piezoelectric plate 52, and the resonance electrode 54 extends to the other end in the longitudinal direction of the piezoelectric plate 52.
In the conventional piezoelectric resonator 51, by applying an alternating electric field between the resonance electrodes 53 and 54, a fundamental mode of a thickness shear vibration is excited, and the fundamental mode is confined or trapped in a vibration portion where the resonance electrodes 53 and 54 are opposed to and overlap each other.
In the conventional piezoelectric resonator 51, the vibration portion is located in the middle of the longitudinal dimension of the piezoelectric resonator 52.
There are cases where the piezoelectric resonator 51 is constructed as a piezoelectric component in which lead terminals are joined to the resonance electrodes 53 and 54 and in which the piezoelectric component with the lead terminals joined thereto is covered with a finishing resin excluding the tip of the lead terminals. Also, there are cases where the piezoelectric resonator 51 is constructed as a piezoelectric component in which the piezoelectric resonator 51 is joined to a substrate of a case with a space provided over the substrate so as not to prevent the vibration portion from vibrating and in which the piezoelectric resonator 51 is covered by a metal cap.
In either structure, the piezoelectric resonator 51 is connected to the lead terminals or the electrodes on the substrate of a case by soldering. Accordingly, during packaging or hardening and contraction of the finishing resin, an external stress may be applied in the longitudinal direction of the piezoelectric plate 52. As a result, the polarization axis of the piezoelectric plate 52 is bent, and accordingly, unwanted ripples are generated in the frequency band between the resonant frequency and the antiresonant frequency.
Particularly, when the thickness of the piezoelectric plate 52 is reduced and the width of the piezoelectric plate 52 is reduced in order to realize a piezoelectric resonator 51 having a smaller size, the above-mentioned ripples are more easily generated. Because of this defect, the piezoelectric resonator 51 could not be reduced in size.
In order to overcome the problems described above, preferred embodiments of the present invention provide a piezoelectric resonator that is constructed to use a fundamental wave of a thickness shear vibration mode and such that, even if a stress is applied to a piezoelectric plate of the resonator, the generation of unwanted ripples between the resonant frequency and the antiresonant frequency is prevented and the piezoelectric resonator can be made much smaller that conventional resonators.
According to a preferred embodiment of the present invention, an energy-trap type piezoelectric resonator utilizing a fundamental wave of a thickness shear vibration mode includes a substantially rectangular piezoelectric plate which is polarized in the longitudinal direction, and first and second resonance electrodes disposed on first and second main surfaces of the piezoelectric plate and arranged to define an energy trap vibration section and to face each other with the piezoelectric plate disposed therebetween, wherein the energy-trap vibration section is arranged to be asymmetrical with respect to the center in the longitudinal direction of the piezoelectric plate.
In the piezoelectric resonator according to various preferred embodiments of the present invention, a relationship 3txe2x89xa6xcex94Lxe2x89xa65t is preferably satisfied, where L is the length of the piezoelectric plate, t is the thickness of the piezoelectric plate, and xcex94L is the distance between the center in the longitudinal direction of the piezoelectric plate and the center in the longitudinal direction of the vibrator portion.
In the piezoelectric resonator according to various preferred embodiments of the present invention, the first resonance electrode is arranged to extend to one end in the longitudinal direction of the piezoelectric plate and the second resonance electrode is arranged to extend to the other end in the longitudinal direction of the piezoelectric plate.
In the piezoelectric resonator according to various preferred embodiments of the present invention, first and second lead terminals are electrically connected to the lead-out portions of the first and second resonance electrodes, respectively, and, excluding the tips of the first and second lead terminal, the rest of the piezoelectric resonator is covered by finishing resin such that a space is defined around the vibration section of the resonator so as not to prevent or hinder the vibration section from vibrating.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the detailed description of preferred embodiments thereof with reference to the drawings.